MOVEMENTS OF DAPHNIA 247 



at normal temperature and 2' 46" after heating. Also, the 

 averages of all the observations show a higher rate of speed at 

 the higher temperature. This effect is produced in spite of the 

 fact that the phototaxis is lessened by the increase of tempera- 

 ture. In this case then we have a definite increase of locomotor 

 activity by increase of temperature. 



Exp. 16 



Effect of heating on rate of locomotor activity 



Time in minutes and seconds required to travel 80 cm. toward a weak light. 



Temperature, C... 20.5° 25.5° 20.5° 25.5° 20.5° 



Individual A 3' 23" 2' 50" 1' 55" 2' 23" 1' 45" 



B 2' 20" 2' 05" 1' 05" 1' 30" 1' 45" 



C 5' 00" 3' 00" 3' 00" 2' 25" 3' 15" 



D 4' 55" 3' 20" 4' 40" 3' 20" 4' 55" 



E 4' 35" 2' 35" 3' 05" 1' 55" 2' 45" 



Averages 4' 02" 2' 46" 2'54" 2' 18" 2' 39" 



Average speed before change of temperature, 4' 02". 



Average speed on first heating, 2' 46". 



Average speed at 25.5° C, 2' 32". 



Average speed at 20.5° C. after heating, 2' 46". 



On cooling daphnids from a normal temperature of 20° C. 

 down to 15° C. there is also an increase of speed of motion 

 toward a weak light and the averages show very definitely a 

 faster rate of speed at the low temperature than at the normal. 

 However, the decrease of temperature increases the positive 

 phototaxis and the increase in speed of travel may be due to 

 this cause rather than to the lower temperature directly in- 

 creasing the locomotor activity. 



Experiment 17 shows the effect of continued exposure to a 

 temperature of 25.5° C. This indicates that while the activity 

 is increased when the temperature is first raised, after a short 

 time the speed of travel decreases and soon the animal travels 

 more slowly than at the normal temperature. It can be directly 

 observed that a continued high temperature causes many of the 

 daphnids to become very weakly positive and to show many 

 erratic movements, even going at times directly away from the 

 light. Many also grow very feeble and show a tendency to 

 settle to the bottom of the dish. This is a real decrease of 

 locomotor activity. However, other individuals show a con- 

 tinued high rate of locomotion after a lengthy exposure to the 

 higher temperature. 



